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Bad Physics, Bad Investment

Friday, January 11, 2013

A typical crank. Image: Hoangquan hientrang via wikimedia

Investors beware, when something sounds too good to be true, it usually is. Right now an entrepreneur is asking for money to manufacture bicycle parts that he says will give cyclists more power, even though the way he claims it works is contradicted by the laws of physics.

The cranks of a bicycle are what connect the pedals to the front gears. They're lever arms that cyclists exert a force onto the end of, through the pedals, in order to turn the front gears. The front gears pull the chain which then spins the rear wheel, sending the bike speeding along.

Z-Torque cranks. Image from z-torque.com

Just about all the cranks on the market are a straight line from the pedal to turning radius. However a company called Z-Torque claims that their cranks give cyclists more power just by changing the crank arms into a bent shape. The problem is that physics doesn't work like the company claims it does.

The inventor, Glenn Coment, claims that his zig-zag design gives peddlers more leverage resulting in more power, but keeps the pedals at the same distance away from the center of rotation. However even a basic understanding of how levers work would show that this is impossible.

Glenn's nephew Jason is licensing the design to make them out of carbon fiber. In the fundraising video, he gives a brief physics lesson about how his cranks supposedly work.

“Pedaling your bike is similar to using a wrench to tighten or loosen a bolt. Any mechanic will tell you if you need more torque to loosen a stubborn bolt, just go get a bigger wrench. That’s because by moving your applied force further away from the pivot point, you gain more leverage," Jason Coment said.

So far he's right. Torque is the twisting force a rotating lever or wheel exerts on its axis. Increasing the length of a lever arm, like a wrench, exerts more torque on its bolt. The longer the lever arm, the more torque is produced by the same amount of force pushing on the end of the lever.

A "second class lever" is the same kind of lever as a crank.

The same would be true with bicycle cranks, which are fundamentally the same as levers. One could get more torque with longer cranks, but the cyclist would have a greater distance around to pedal. The problem of course is human legs are only so long, and there's no real comfortable way to make the cranks significantly longer without people being unable to reach the pedals.

"That’s why with the patented Z-Torque bicycle crank we have solved that problem for you," Jason Coment claims. "You see we extended the crank arms past the length of a standard crank, giving you more leverage, but we then brought pedal back towards the axle to keep your rotation at the same diameter.”

He incorrectly claims that a cyclist can get more torque by having a crank arm that's "longer" but bends back towards the center, keeping the pedals the same distance away from the axis as a traditional straight crank. Levers don't work like that. It doesn't matter what shape the lever arm is, it only matters how far away the pedal is from the center of rotation.

“Having a wiggly line between one and the other doesn’t do anything about the torque," said David Gordon Wilson, an emeritus professor of engineering at MIT and author of Bicycling Science. “The tortuous form of the crank is just crazy.”

He said one could imagine welding a piece of aluminum straight between the pedal and the axis of rotation on the angled cranks. The leverage of the cranks would be the same whether the crank arm is straight, angled curved, or any other shape. The only thing that matters for leverage is how far the pedals, the source of the exerted force, is from the axis of rotation.

“This Z-crank has no redeeming features whatsoever,” Wilson said.

Coment's design isn't new, designs for curved or angled cranks have been around since the 1930s. He's had prototypes for his Z-Torque cranks since 1995, a patent on them since 1999, and a website selling aluminum versions since at least 2009. Recently he's been trying to expand and make carbon fiber versions. In September of last year, Coment launched a Kick-starter project to raise $50,000 to buy tools and equipment, but missed his goal by more than $47,000. He's now trying again, on a different crowd-funding website, "Rock the Post" with a more modest $7,500 goal.

The website also claims that the cranks give riders "Less perceived effort to pedal." In the medical world, I think they would call that a "placebo."

83 Comments:

Anonymous said...

These cranks have been tested for over 20 years now. they do not give you more leverage. All they do is allow you to push down with all your effort as soon as top dead center is passed. The original design tested by Wake Forest University and the Submarine club of Florida Atlantic University produced an increase at the rear wheel,(or prop ), of over 20% compared to a standard straight crank arm. This allowed FAU to set a record for their sub at the International Submarine Races in 2005. This was after making 15 attempts at the absolute speed record with standard straight cranks and not being able to even claim an award. A standard crank has a so called dead spot between TDC and 60 degrees where little or no positive return is generated from your effort. That is why you learned to let the crank on your bike to swing out before pushing down as a kid. Just imagine the increase in power by starting to push down sooner and for a longer period of time.

Saturday, January 7, 2017 at 6:56 PM

Anonymous said...

I have these on my bike, I am lazy and live in a hilly area. I get home from work, prop up the back wheel onto a stand and pedal for hours. That way, all of the stored energy in the Z crank lets me ride in leisure on the weekends, NO PEDALING REQUIRED!

Monday, March 28, 2016 at 6:37 PM

eagledove9 said...

I'm wondering, what if the metal *bends* when you push your foot against it, then springs back into place, and when it springs back, it transfers energy somehow? It would be similar to putting another joint at the bent part of the Z, with a spring tied to it so that it would spring back after you pushed your foot against it. The spring holds force and... I can't explain. It's like the pedal extends like a spring when you push your foot on it, then springs back into place. If you timed the spring-back moment at exactly the right position, it might be, like, in the position where it was going around the bottom of the circle, and would therefore send it around even faster without any effort put on it. You'd just give the pedal a little tiny bit of a push, which would extend the spring. It would move to the downward side, then spring back inwards, right at the lowest point of the circle. I don't know, it could be all wrong, I'd have to try it but I don't have the tools or the motivation for such things.

Thursday, July 23, 2015 at 11:08 PM

Anonymous said...

No, because (ignoring flexure, added mass and the new and exciting failure mode) this is identical to a straight crank.

Imagine if you decided to make this stronger without modifying the mechanical properties by joining the connection point for the pedal to the spindle. You then have a normal crank with this extra metal stuck to the side of it. Which you remove, to save weight. Now you have a normal crank.

Monday, August 12, 2013 at 10:56 AM

Anonymous said...

You know what would be awesome? 15kg flywheels on bikes. They'll get you over the hills.

Monday, August 12, 2013 at 10:47 AM

Doctor FTSE said...

There's a much more efficient way of getting your bike to go faster/make the effort of biking feel easier.It's called "TRAINING." Try two/three hundred miles per week, 50% cruising, 50% 'flat out."

(And I am a robot, so why do I have to prove it to some idiot who wants me to waste my time deciphering hs idiot word?)

Sunday, April 7, 2013 at 5:06 AM

Anonymous said...

Best reply here.

Wednesday, March 13, 2013 at 1:58 PM

Anonymous said...

-_- Not even on a physics article.......Just couldn't resist could you.

Thursday, February 7, 2013 at 6:24 PM

SandyCat said...

From this perspective, steam locomotives are like huge powerful bicycles with steam-driven "pedals" {main rods]. The potentially crippling dead spot was countered with a 90-degree offset between the R & L main rods. If a Z-shaped eccentric crank would have afforded more power, this would have been the time & place for it with money riding on the discovery. (And toss eccentric crank into delightful stack of puns in this wonderful thread.)

Saturday, January 19, 2013 at 12:22 AM

Z-Torque said...

Unfortunitly Jason has it all wrong as to how this crank works. The crank does not give you more leverage. It gives you a high pressure at the chain wheel for a longer period of time. A straight traditional crank during the, "dead spot",(the first 60 degrees of rotation),wastes most of your effort by sending it down the arm to the axle instead of turning the assembly. Remember when you first learned to ride a bike. If you pushed down to soon you picked yourself right up off the bike. Look at the Z-torque in the picture.The long part attached to the axle of the bottom bracket is the real working crank arm. An arm moving out of the"dead spot" when the pedal is at Top dead center. When the small part of the crank arm that runs from the end of this "real" crank arm to the pedal is made at the correct angle and length,it directs your effort away from the axle of the bottom bracket.This allows you to apply all your effort sooner. Increasing your momentum and power. A Z-torque only has a "dead Spot" right at Top Dead Center,(TDC),and Bottom Dead Center,(BDC). Anywhere else you can push down or pull up with all your effort. IT' LIKE TAKING A LONGER SWING WITH A BASEBALL BAT. MORE OF YOUR EFFORT IS EFFICIENTLY TRANSFERRED TO THE BICYCLE. I hope this clears up some of your questions.

Friday, January 18, 2013 at 9:47 PM

Anonymous said...

I actually had this same idea several years ago. Then I built a simple prototype out of wood, tested it, and saw that it didn't do squat. Apparently my second thought should have been, "Don't test it, just build it and charge money for it."

Tuesday, January 15, 2013 at 7:40 PM

Anonymous said...

Guess he should go pedal his bs somewhere else...

Monday, January 14, 2013 at 9:15 PM

Anonymous said...

Biopace was great for efficiency, but terrible on the mechanics of the human body in order to "spin" since some of the pros liked spinning. I still have a biopace ring on my mt. bike crank (low gear of course).

Hence Oval rings we're the compromise.

In the end, round was the best compromise.

This Z crank has nothing to do with more leverage or being more efficient in the human anatomy dept. I have a bridge to sell as well...

Monday, January 14, 2013 at 2:25 PM

Anonymous said...

"The inventor, Glenn Coment, claims that his zig-zag design gives peddlers more leverage"

From that alone it is obvious crankery. Only if the pedal, the place where the force is applied, is further from the pivot will you get more leverage. Here's a thought experiment: Take a regular crank and add to it a "z-arm" so it is like a triangle? Do you get more leverage? Take away the original crank arm so you are left with just the z. Do you now get more leverage?

Monday, January 14, 2013 at 12:14 PM

carolinekuhn said...

I would love to try out your marvelous IDEA!!! But I am in the Netherlands, so I guess I do not qualify for that. I wish you the best luck. I love people with passion for what they do!! Caroline Kuhn

Sunday, January 13, 2013 at 4:06 PM

Anonymous said...

Sigh

Sunday, January 13, 2013 at 2:31 PM

Anonymous said...

Love 'em or hate 'em, Biopace rings had real benefits that a rider could use.

Saturday, January 12, 2013 at 9:17 PM

Anonymous said...

That's why I always set my saddle up higher than the handlebar. I'm always going downhill, even when I'm going uphill.

Saturday, January 12, 2013 at 9:13 PM

Anonymous said...

I'm just gonna point out some common sense here....anytime something gives, its going to absord energy that its not going to return at 100% to the system. Even if we ignore physics, the most this crap would do is steal power from a cyclist.

this is why I detest mountain bikes for anything but mountain biking.

Saturday, January 12, 2013 at 8:30 PM

Jonathan Gahan said...

This thing has, in fact, been granted an actual US patent, not a design patent. Patent #5,899,119 granted 4 May 1999. http://www.google.com/patents/US5899119

Best part is that "flexing" of the crank arm is specifically referenced in the patent as the enabling factor.

Saturday, January 12, 2013 at 5:09 PM

Anonymous said...

Don't knock the square wheel, it's proven to work... given the right road surface.

That is a good mental model. To expand on it. Imagine if the pedal was extended all the way back past the center. Now, step on the pedal. What direction does the chain ring rotate? Counter-clockwise! (aka, the wrong direction.)

Saturday, January 12, 2013 at 2:49 PM

Anonymous said...

Z-Tourque's next product: square wheels for better traction when climbing or descending. More tire on the road means better braking!

Saturday, January 12, 2013 at 2:34 PM

Anonymous said...

Assuming their data isn't an outright lie, is it possible that the benefits they claim to be seeing in testing actually have no bearing on the "crank" physics of the crank and more to utilizing a slightly differed combination of muscle groups because of the different placement of the pedal at certain points in each stroke?

Saturday, January 12, 2013 at 2:30 PM

Anonymous said...

I'm sure that Obama will give him your tax dollars. This guy obviously doesn't understand even basic physics, but that hasn't stopped the idiot and chief from giving away your tax dollars before. Why should it stop him from doing it now... Of course after bankruptcy we'll be told by the media that no one could have foreseen this inevitable failure...